Coronavirus: Antibody tests would put up to quarter of those told they were immune at risk of infection, government advisers warn

Scientists have warned the government that even highly accurate antibody tests for coronavirus could leave more than a quarter of people who were told they were immune at risk of infection.
New documents published today reveal the problem exists even for antibody tests that are 98 per cent accurate. Even if the tests are 99 per cent accurate almost 10 per cent of people who were told they were immune would be put at risk.
The conclusions, considered by the government’s Scientific Advisory Group for Emergencies, or SAGE, suggest the much anticipated development of home antibody kits may not be the route to exiting the coronavirus lockdown many had hoped.
The problem comes from the number of patients who may receive a false result even though the tests are so accurate.
The SAGE paper says the performance of the tests depend on how many people who actually have antibodies test positive, this is called the sensitivity, and how many people without antibodies test negative, which is described as the specificity.
Alongside this is the spread of the disease within society and the number of people who actually have antibodies – the prevalence.
The SAGE document said for a test which is more than 98 per cent accurate and where 5 per cent of the population are thought to have antibodies, a test would lead to 68 people being told they had had the virus and were immune.
Of these, 19 people, or 28 per cent would not have antibodies and would be put at risk of infection if allowed to go back to work or mix in public with those carrying the virus. At least one person out of 1,000 would be wrongly told they had not had the virus and would be forced to stay in lockdown.
If the spread of the virus increases to 10 per cent of the population, the results mean 116 people would be told they were immune, but of those 18, or 16 per cent, would not actually have antibodies and susceptible to infection.
The problem persists even if the test is 99 per cent accurate. With a 5 per cent prevalence of the virus, 16 per cent of those who test positive for antibodies would be incorrect falling to 8 per cent if the virus has infected 10 per cent of the population.
The Medicines and Healthcare products Regulatory Agency, which regulates medical devices, has set a minimum standard of 98 per cent accuracy for antibody test kits.

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